1. Field of the Invention
This invention relates to a braking force control method and device for a strip-shaped material feeding device, which are preferred when applied to a feeding device or the like of a rotary printing press.
2. Description of the Related Art
Examples of a braking device in a feeding device of an offset rotary press include those as shown in FIGS. 22 and 23 (see Japanese Patent Application Laid-Open No. 1995-61661; hereinafter referred to as patent document 1).
In the printing press, a web roll 103 is rotatably supported between a pair of turret arms 100a and 100b via a taper cone 101 and a mechanical chuck 102. According to the braking device, the web roll 103 is braked by an air brake 104 when tension is controlled in a routine operation or when the printing press comes to a sudden stop. The air brake 104 is of an ordinary type pressing brake pads 108, which are supplied with pressurized air controlled by an electro-pneumatic regulator 107, against opposite side surfaces of a brake disk 106 secured onto a rotating shaft 105 supporting the web roll 103, thereby applying a braking force in a rotating direction to the rotating shaft 105.
In tension control during a routine operation, for example, a control torque command according to the diameter of the web roll 103, which has been computed, is outputted by a sequencer 109, as an air pressure, to the air brake 104 via the electro-pneumatic regulator 107 to give tension to a web W rolled off. Based on a value detected by a tension sensor 111 (detects the tight side of tension) in a tension roller 110 and a position detected by a potentiometer 113 (detects the loose side of tension) in a dancer roller 112, feedback control is exercised.
Alternatively, as described in Japanese Patent Application Laid-Open No. 1994-227722 (hereinafter referred to as patent document 2), the regenerative braking force of a web accelerating motor, as well as the braking force of the braking device, is utilized such that the braking force of the braking device is used as a main braking force, and the regenerative braking force of the web accelerating motor is used as an aid only when the required braking force is greater than the braking force of the braking device.
With the braking device of patent document 1, the air brake 104 is actuated for tension control during a routine operation, or at the time of sudden shutdown of the printing press. Thus, the properties of the brake are changed by the surface deterioration of the brake pad 108 due to change with time or the carbonization of the brake pad 108 due to heat generation. As a result, variations occur in the control output-torque characteristics (see FIG. 4(b)) of the air brake 104, thus making accurate control impossible. Also, periodical inspection and replacement of the brake pad 108 become necessary. This has posed the problems that an operator is burdened and the efficiency of work is decreased.
With the braking device of patent document 2 as well, drawbacks similar to those of patent document 1 occur when the braking force of the air brake is used as the main braking force. That is, the properties of the brake are changed by the surface deterioration of the brake pad due to change with time or the carbonization of the brake pad due to heat generation. As a result, variations occur in the control output-torque characteristics (see FIG. 4(c)) of the air brake, thus making accurate control impossible. Also, periodical inspection and replacement of the brake pad become necessary. This has posed the problems of burdening the operator and decreasing the work efficiency.
In the braking device of patent document 2, it is conceivable to use only the regenerative braking force of the web accelerating motor, for the purpose of tension control during a routine operation, or at the time of sudden shutdown of the printing press. In this case, a motor with a very high capacity is required, presenting the unexpected problem of poor economy.